Power controllers have been utilized in battery backed-up systems to switch in a backup battery in the event that a primary power supply fails. Specifically, a comparator is utilized that compares the primary power supply voltage to a reference voltage. In the event that the voltage of the primary power supply falls below this reference voltage, a signal is generated that causes a backup battery to be switched in. Whenever the voltage of the primary power supply rises above the threshold voltage, the primary power supply is again switched over to the output of the power controller. However, these systems work on the premise that the voltage of the primary power supply voltage and the threshold voltage at which the decision is made is always above the voltage level of the battery. These systems generally utilize a lithium battery that has a voltage of around three volts, with the primary power supply having a voltage of around five volts. The threshold for making the determination as to the primary power supply being low is approximately 4.5 volts. One disadvantage to present systems is that they do not take into consideration low voltage operating modes, wherein the primary power supply voltage is approximately equal to the backup battery voltage. In such a situation, the fully charged battery could have a voltage level higher than the power supply voltage.
Whenever a system utilizes a backup battery that may have a voltage higher than the primary power supply voltage, there always exists a possibility that, during normal operating mode with the primary power supply, a current path will exist between the backup battery and the primary power supply, thus draining current from the backup battery. This occurs due to the fact that the switching transistors utilized in the semiconductor structure typically share a common source with the drains of the switching transistors connected to the respective power supply. Typically, the sources and drains of transistors are fabricated from one conductivity type semiconductor material whereas the channel regions and the surrounding semiconductor material is fabricated from a second and opposite conductivity type of material in a common region. If the drain of the transistor associated with the primary power supply were disposed at a lower voltage than the drain of the transistor associated with the backup battery, a forward biased PN junction could exist. For example, a switching transistor with P-channel transistors fabricated in a PMOS type device would require an N-type well being formed within a P-type substrate. Within the N-type well, sources and drains would be fabricated from P-type impurity implants. Typically, an N+ impurity implant is utilized within the well to provide a contact region to allow a bias to be supplied thereto. This region is typically connected to the power supply voltage provided to the system. If this voltage were lower than any of the P-type implants, such as the drain implant of the transistor associated with the backup battery during operation from the primary power supply, a forward biased PN junction would result at the interface between the drain implant associated with the backup battery power supply and the well, this being connected directly to the N+ implant that is connected to the power supply. As a result, a current path would be provided from the battery, which is at a higher voltage, to the primary power supply, thus draining the backup battery into the primary power supply, causing an unnecessary loss of current.